For Sales force Lightning :
User can send the bulk messages using Opportunity tab:
To achieve this:
1. Click the Opportunity tab & select All Opportunities/My Opportunities option from the Opportunity drop down on top left corner (then click Go button if user is in salesforce classic mode).
2. Select the desired Opportunities by selecting the respective check boxes where message is to be sent.
3. Click Bulk SMS button on top right corner of the page.
4. All related Contact corresponding to the selected Opportunity will be displayed on the next page.
5. From here, user can Send the messages as desired.
The success message will be displayed once the message is sent.